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arXiv:1107.4746 · Physical Review A

Optimized coupling of cold atoms into a fiber using a blue-detuned hollow-beam funnel

Jérôme Poulin, P. S. Light, Raman Kashyap, Andre Nicholas Luiten

Published Nov 7, 2011• 13 citations

Abstract

We theoretically investigate the process of coupling cold atoms into the core of a hollow-core photonic-crystal optical fiber using a blue-detuned Laguerre-Gaussian beam. In contrast to the use of a red-detuned Gaussian beam to couple the atoms, the blue-detuned hollow beam can confine cold atoms to the darkest regions of the beam, thereby minimizing shifts in the internal states and making the guide highly robust to heating effects. This single optical beam is used as both a funnel and a guide to maximize the number of atoms into the fiber. In the proposed experiment, Rb atoms are loaded into a magneto-optical trap (MOT) above a vertically oriented optical fiber. We observe a gravito-optical trapping effect for atoms with high orbital momentum around the trap axis, which prevents atoms from coupling to the fiber: these atoms lack the kinetic energy to escape the potential and are thus trapped in the laser funnel indefinitely. We find that by reducing the dipolar force to the point at which the trapping effect just vanishes, it is possible to optimize the coupling of atoms into the fiber. Our simulations predict that by using a low-power (2.5 mW) and far-detuned (300 GHz) Laguerre-Gaussian beam with a 20-$\ensuremath{\mu}$m-radius core hollow fiber, it is possible to couple 11% of the atoms from a MOT 9 mm away from the fiber. When the MOT is positioned farther away, coupling efficiencies over 50% can be achieved with larger core fibers.

DOI: 10.1103/physreva.84.053812

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We theoretically investigate the process of coupling cold atoms into the core of a hollow-core photonic-crystal optical fiber using a blue-detuned Laguerre-Gaussian beam. In contrast to the use of a red-detuned Gaussian beam to couple the atoms, the blue-detuned hollow beam can confine cold atoms to the darkest regions of the beam, thereby minimizing shifts in the internal states and making the guide highly robust to heating effects. This single optical beam is used as both a funnel and a guide to maximize the number of atoms into the fiber. In the proposed experiment, Rb atoms are loaded into a magneto-optical trap (MOT) above a vertically oriented optical fiber. We observe a gravito-optical trapping effect for atoms with high orbital momentum around the trap axis, which prevents atoms from coupling to the fiber: these atoms lack the kinetic energy to escape the potential and are thus trapped in the laser funnel indefinitely. We find that by reducing the dipolar force to the point at which the trapping effect just vanishes, it is possible to optimize the coupling of atoms into the fiber. Our simulations predict that by using a low-power (2.5 mW) and far-detuned (300 GHz) Laguerre-Gaussian beam with a 20-

\ensuremath{\mu}

m-radius core hollow fiber, it is possible to couple 11% of the atoms from a MOT 9 mm away from the fiber. When the MOT is positioned farther away, coupling efficiencies over 50% can be achieved with larger core fibers.

Summary form only given. Extracting cold atoms from a magneto-optical trap (MOT) has been the subject of central interest in recent years. This has seen the emergence of the area of atom guiding along hollow core optical fibers and also along light beams. The guiding and the extraction of cold atoms is of central importance in generating bright, high flux atomic sources for various experiments in atom optics. In this paper we describe and experimentally realize a scheme for extracting cold atoms directly from a magnetooptical trap along a Laguerre-Gaussian light beam.

We propose an atomic funnel consisting of a dark hollow, divergent laser beam and an evanescent light in a short micron-sized hollow optical fiber. The cold atoms extracted from a magneto-optical trap experience effective Sisyphus cooling in the dark hollow beam. We calculate the potential barriers for atomic guiding in the evanescent light and the dark hollow beam, and analyze the atomic losses during the guiding and funneling process. Our results show that the equilibrium three-dimensional root-mean-square momentum of about $4.5\ensuremath{\Elzxh}k$ (temperature $\ensuremath{\sim}2.5\ensuremath{\mu}\mathrm{K}$) and the funneling efficiency of about 95% could be obtained. A low-velocity, intense coherent atomic beam can be generated in this funnel scheme and its potential applications in atom optics are briefly discussed.

Cold atoms trapped and guided in hollow-core photonic crystal fibers provide a scalable diffraction-free setting for atom–light interactions for quantum technologies. However, due to the mismatch of the depth and spatial extension of the trapping potential from free space to the fiber, the number of cold atoms in the fiber is mainly determined by the loading process from free space to waveguide confinement. Here, we provide a numerical study of the loading dynamics of cold atoms into a hollow-core photonic crystal fiber. We use the Monte Carlo method to simulate the trajectories of an ensemble of cold atoms from free space trapping potential to optical potential inside a hollow-core fiber and calculate the temperature, loading efficiency, and geometry of the ensemble. We also study the noise sources that cause heating and a loss of atoms during the process. Our result could be used to design and optimize the loading process of cold atoms into a hollow-core fiber for cold atom experiments.

Summary form only given. The optical guidance of cold atoms by the hollow laser beam (HLB) with blue detuning has been well understood experimentally and theoretically (Xinye Xu et al, 2000). Based on this clear understanding and the development of an efficient atomic guiding, we have performed the experiment of optically guided atomic fountain by using a blue-detuned cylindrical HLB. An HLB was generated from a micron-sized hollow optical fiber and collimated by the objective lens and propagates downwards to the center of a magneto-optical trap (MOT). We have demonstrated tenfold enhancement of the atom-funneling efficiency for the HLB-guided atomic fountain, which may lead to improved performance of atom optical experiments based on the atomic fountain such as the Rb atomic clock.

Atom guiding in an atomic funnel is very useful for many interesting atom optical experiments requiring the precise control of atomic motion. We have obtained a doughnut-shaped laser beam by coupling a Ti:sapphire laser into a hollow core optical fiber and collimating the output laser beam of the fiber with a microscope objective lens. Using such a doughnut-shaped laser beam [2 mm inner diameter and a 5 mm outer diameter at the trapping position (about 1m away from the objective lens)l, we have guided the trapped cold Rb atoms in the atomic funnel.

Laser-cooled atoms from a low-velocity atomic source are guided in a hollow-core optical fiber using the evanescent-wave dipole force from blue-detuned laser light launched into the glass region of the fiber. The transverse velocity of the guided atoms corresponds to a temperature of $50 \ensuremath{\mu}\mathrm{K}.$ We achieve a maximum flux through a 23.5-cm-long fiber of 590 000 atoms/second with a laser power of 55 mWatts at a detuning of 6 GHz. With larger detunings of 40 GHz, spontaneous emission from the atoms inside the fiber can be suppressed and the atom's internal-state population is preserved. We identify two major loss mechanisms for the guiding process and discuss possible solutions.